Start writing doc about GitLab architecture for developers

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

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+# GitLab project architecture
+
+GitLab project consists of two parts: GitLab and GitLab shell.
+
+## GitLab
+
+Web application with background jobs workers. 
+Provides you with UI and most of functionality.
+For some operations like repo creation - uses GitLab shell.
+
+Uses: 
+ * Ruby as main language for application code and most libraries. 
+ * [Rails](http://rubyonrails.org/) web framework as main framework for application.
+ * Mysql or postgres as main databases. Used for persistent data storage(users, project, issues etc). 
+ * Redis database. Used for cache and exchange data between some components.
+ * Python2 because of [pygments](http://pygments.org/) as code syntax highlighter.
+
+## GitLab shell
+
+Command line ruby application. Used by GitLab through shell commands.
+It provides interface to all kind of manipulations with repositories and ssh keys.
+Full list of commands you can find in README of GitLab shell repo.
+Works on pure ruby and do not require any additional software.